1. Field of the Invention
The invention relates generally to battery charging systems, and more particularly to a battery charging method and apparatus for charging lead acid batteries.
2. Description of the Prior Art
In order to charge lead acid batteries in an efficient and timely manner, a three phase charging process consisting of constant current charge, constant voltage charge and float voltage charge advantageously is used.
FIG. 4 shows typical voltage and current profiles of a known three phase battery charger. Phase 1 is the constant current phase. In phase 1 the traditional battery charger forces a constant current into the battery by setting a reference in the charger current control loop and feeding the battery input current into the charger control loop. During phase 1, the battery voltage increases as more charge is put into the battery. When the battery voltage reaches a predetermined value, VC, the battery charger ends the constant current charging phase and enters phase 2, the constant voltage charge phase. At the end of phase 1, the battery is nominally 85% charged.
In phase 2, the traditional battery charger will shift from forcing constant current into the battery to regulating the battery voltage at VC +/-1%. This is accomplished by setting a reference in the charger voltage control loop and feeding back the battery voltage into the charger feedback loop. During phase 2 charging, traditional battery chargers disconnect the battery input current from the charger control loop. In phase 2 charging, the battery input current decreases exponentially because the amount of charge required to keep the battery voltage at VC decreases. When the current entering the battery reaches a predetermined value, IX, typically about 1/10 of the phase 1 current, the battery charger ends constant voltage charging and begins float voltage charging or phase 3. At the end of the phase 2, constant voltage charges the battery to 100% charged.
In phase 3, the traditional battery charger continues to regulate the battery voltage, but now the regulation is at a second predetermined voltage Vf that is usually lower than VC from phase 2 constant voltage charging. Implementation of the charger control loop is identical to phase 2 implementation. The purpose of phase 3 charging is to offset the battery's self-discharging mechanism and the amount of current discharged in the battery pack circuits. Phase 3 charging will continue uninterrupted until the battery is discharged by a system request for energy back-up.
In the constant voltage phase 2 and float charging phase 3 the battery voltage must be regulated. In the known prior art arrangement, the battery voltage was regulated by connection to an analog feedback loop of a switching power supply. The analog feedback loop provides feedback in real time.
Due to the nature of lead acid batteries, traditional battery chargers can become unstable during phase 2 and phase 3 charging. Controlling the voltage of a lead acid battery with a switching power supply charger creates stability problems for the charger feedback loop. A battery is very slow to react to changes in charge rates. This results in a large phase lag to the charger control loop. Compensation for this phase lag can be provided in the charger feedback loop design if the battery circuit characteristics do not change significantly. However, lead acid battery circuit characteristics change appreciably with battery charge state, battery age and battery temperature. These changes are not completely predictable. As a result the known battery charger control loop can become unstable. An unstable battery charger control loop will result in batteries not being charged to the correct specifications and can shorten battery life in the field.
A need exists for a method and apparatus that prevents an unstable battery control loop and that is simple to implement.